The present invention relates to a hydraulic power steering system for automotive vehicles. Specifically, the present invention relates to improvements in hydraulic fluid pumps used in power steering systems to reduce parasitic horsepower losses in the pump thereby saving energy for driving the pump.
Hydraulic power steering systems for automotive vehicles employ, as a source of hydraulic fluid, a pump driven by the vehicle engine. The horsepower required to drive the pump is proportional to the product of the rate of flow and the discharge pressure of the pump. The displacement of the pump must be great enough to supply the flow demand of the steering gear during steering maneuvers when the engine operates at low speed.
As the pump speed increases with the vehicle speed, the flow delivery increases. But since the horsepower consumption of the pump is proportional to both flow and pressure at the outlet of the pump, a feasible way to reduce the pump free-flow horsepower loss is by reducing the discharge pressure. This is in contrast to the concept employed in the prior art systems where reducing the flow by means of a flow control valve reduces horsepower loss.
U.S. Pat. No. 5,112,199, issued to Otaki et al. on May 12, 1992, discloses a pump having a flow control valve to reduce the flow when the engine operates at a high speed. With the flow control valve, the volume of hydraulic fluid far in excess of the flow requirement is returned from the working chamber of the pump to the pump inlet. A similar pump is disclosed in U.S. Pat. No. 5,098,259 issued to Ohtaki et al. on Mar. 24, 1992.
A power steering pump flow and pressure control valve disclosed in U.S. Pat. No. 4,135,436 issued to Duffy on Jan. 23, 1979 employs the concept of reducing the pump discharge pressure. According to this prior art, using a variable orifice at a location between the pump discharge and the power steering gear inlet reduces the discharge pressure of the pump. The variable orifice includes an orifice pin that is carried by a flow control valve spool. The position of the pin may be changed relative to the spool in response to changes in the pressure upstream of the orifice. With this arrangement, when the pressure is increased in response to an increased torque demand, the orifice pin will become adjusted to increase the effective size of the flow control orifice thereby allowing the flow to increase to a higher level during high-pressure operation. In contrast, the pressure is reduced when the flow is reduced as the orifice pin returns to a high restriction position relative to the valve spool.
A power steering system disclosed in U.S. Pat. No. 5,471,838 issued to Suzuki et al. on Dec. 5, 1995 saves horsepower required to drive the pump. According to this known power steering system, a supply passage between the pump outlet and the steering gear inlet is provided with two orifices in series, namely, a metering orifice and a control orifice upstream of the metering orifice. A flow control valve responsive to the pressure difference across the metering orifice returns the excess volume of hydraulic fluid to the pump inlet when a predetermined rate of flow is exceeded. A bypass valve responsive to the pressure difference across the control orifice drains the pressure in a spring chamber of the flow control valve when the pressure difference across the control orifice exceeds a predetermined value. This situation occurs when the pump rotates at a speed higher than a predetermined speed value. Since the flow control valve keeps the pressure difference across the metering orifice almost constant, draining the pressure in the spring chamber reduces the pump discharge pressure thereby reducing the horsepower required to drive the pump. During a steering maneuver, a bypass flow control valve recovers the flow to the steering gear inlet. Turning the steering wheel increases load pressure acting on the steering gear inlet. In response to the increased load pressure, the bypass control valve restricts draining of the pressure in the spring chamber.
U.S. Pat. No. 5,474,145 issued to Haga et al. on Dec. 12, 1995 corresponds to JP-A 7-81593 and discloses a hydraulic power steering system including a pump, a flow control valve and a bypass control valve. A metering orifice is disposed in a supply passage between the pump outlet and the steering gear inlet. The flow control valve responsive to the pressure difference or pressure drop across the metering orifice returns the excess volume of hydraulic fluid to the pump inlet. A spring chamber of the flow control valve is connected to the supply passage at a portion downstream of the metering orifice through a feedback passage provided with a control orifice. The bypass control valve is fluidly disposed between the spring chamber and the fluid reservoir and controls the pressure in the spring chamber. The bypass control valve has a spool, a load pressure admission port and a pilot port. The load pressure admission port is connected to the feedback passage at a portion upstream of the control orifice or the supply passage at a portion between the metering orifice and the steering gear inlet. The pilot port is connected to the feedback passage at a portion downstream of the control orifice. The spool has at one end an enlarged diameter pressure acting area exposed to the load pressure at the load pressure admission port. At the opposite end, the spool has a reduced diameter pressure acting area exposed to the pilot pressure at the pilot port. The bypass control valve has a spring biasing the spool toward the load pressure admission port.
The load pressure increases during steering maneuvers by the operator. When the load pressure increases, the bypass control valve shuts off draining of the pressure in the spring chamber thereby causing the pressure in the spring chamber to increase. In response to the increased pressure in the spring chamber, the control valve closes the bypass passage thereby reducing flow of hydraulic fluid from the supply passage at a portion upstream of the metering orifice to the pump inlet to zero. This increases the flow through the supply passage to the steering gear input.
U.S. Pat. No. 5,577,573 issued to Haga et al. on Nov. 26, 1996 shows the control valve and bypass control valve as an integral part of the pump. To supply less flow requirement the higher the vehicle speed is, a variable orifice is arranged in the place of a fixed orifice as the metering orifice. As the vehicle speed increases, a solenoid-operated rod gradually closes the variable orifice thereby to gradually restrict flow to the steering gear inlet.
According to the known system using the load pressure responsive bypass control valve, if a minimum flow to the steering gear inlet is low, it takes a considerable time until development of the load pressure high enough to activate the bypass control valve upon rapid turning maneuver. This results from consumption of volume of hydraulic fluid by the power cylinder. This delay leads to an undesired slow build-up of power-assist. Thus, the minimum flow is adjusted to a level high enough to minimize the above-mentioned delay. In other words, the minimum flow cannot be adjusted below this level which is high.
According to this known system, when the steering wheel is at rest or in neutral state, the bypass control valve allows flow through the pilot port toward the drainage. Under this condition, hydraulic fluid discharged from the pump must pass through the metering orifice and then the control orifice before reaching the pilot port.
For the previously mentioned reasons, discharge pressure of the pump cannot be lowered to a satisfactorily low level when the steering wheel is in neutral state.